The present invention relates generally to fire detectors, and more particularly concerns fire detectors that sense both ultra-violet (UV) wavelength radiation through a UV channel, and infrared (IR) wavelength radiation through an IR channel. In order to greatly increase performance, the IR channel senses two distinct wavelengths.
Numerous fire detection schemes exist in the prior art which involve the sensing of different combinations of various spectral bands which are emitted from flames. These bands are selected from the UV, visible, and IR spectral regions. A common objective of these various detection schemes is to detect flame in the presence of undesired background radiation sources. These undesired radiation sources include solar radiation, arc-welding, and lightning, and can cause false alarms. It is of course a goal of any fire detection system to avoid false alarms.
Examples of prior art fire detectors include that of U.S. Pat. No. 3,665,440 to McMenamin. McMenamin teaches of a fire detector having two optical sensors; a UV sensor which responds to radiation with wavelengths less than 440 nanometers, and an IR sensor which responds to radiation from 1.0 microns to 2.5 microns. This system could theoretically respond to both hydrocarbon and non-hydrocarbon fuels. Response level to hydrocarbon flames, however, will be significantly less than that of the present invention since hydrocarbon flames show very strong spectral emission near 4.4 microns, as well as emission in the 2.7 micron spectral region. The detection scheme of McMenamin does not respond to radiation near 4.3 microns. Additionally, the scheme of McMenamin includes a UV sensor which is not solar blind since response is allowed for UV wavelengths as long as 400 nanometers. Significant solar emission occurs in the spectral region of 295 to 400 nanometers.
Another advantage of the present invention over McMenamin is that the center wavelength position and bandwidth selected for detection of IR water band emission near 2.9 microns is better matched with the actual wavelength position and bandwidth of IR emission from non-hydrocarbon fuels such as pure hydrogen and hydrazine. McMenamin claims an IR channel centered near 2.5 microns, which is off-center from the region of strongest flame emission. Also, the IR band centered at 2.5 microns in McMenamin will be more responsive to solar radiation than the present invention since solar irradiance at sea level becomes large for wavelengths shorter than 2.5 microns. In addition, the IR bandwidth centered near 2.5 microns as in McMenamin leads to significant loss of signal with off-axis shift of the flame source relative to the detector. This is because of the shift toward shorter wavelength of bandpass which occurs in IR interference filters as a result of off-axis shift.
U.S. Pat. No. 4,199,682 to Spector et al. refers to a fire and explosion detector employing two optical sensors; a UV sensor which responds to radiation with wavelengths less than 300 nanometers and an IR sensor which responds to radiation from 2.5 microns to 2.75 microns.
Spector is similar to the invention of McMenamin except that the UV sensor in Spector is disclosed as having spectral response such that the UV sensor is solar blind. Additionally, the IR sensor in Spector is restricted to response in the 2.5 micron to 2.75 micron region. These differences would make the response of Spector to solar radiation less than that associated with McMenamin. But, as compared with the present invention, Spector would show weaker response to non-hydrocarbon flames because of the narrower bandwidth of IR detected by the IR sensor in Spector and because of the choice of center wavelength for this band in Spector which is not centered on the maximum of the spectral emission of the water band from such flames as hydrogen. As with McMenamin, Spector would be less responsive to hydrocarbon flames than is the present invention. This is due to Spector's lack of response to 4.4 micron emission. Additionally, the existence of water, ice or snow on the window of the IR sensor would render Spector incapable of detecting a fire since they largely absorb the 2.5 micron to 2.75 micron radiation.
Another example of a prior art fire detector is that of U.S. Pat. No. 4,206,454 to Schapira et al. Schapira refers to a flame detector having two optical sensors, an IR sensor which responds to radiation in a narrow band at 4.3 microns and a second IR sensor which responds to a narrow band at 2.7 microns. Schapira provides a means to sense hydrocarbon fires since detection is accomplished in two IR spectral regions, 2.7 microns and 4.3 microns. Hydrocarbon fires show significant levels of IR spectral emission in these two regions. Schapira, however, only produces a response to a fire if the intensity level of the sensor with the 4.3 micron filter exceeds the intensity level of the sensor with the 2.7 micron filter. Non-hydrocarbon fires will not be detected since they have no 4.3 micron emission. Also, it is likely that Schapira could produce a false alarm response when solar radiation is present in combination with a blackbody source (a heated object) with temperature below 1000 degrees Kelvin (.degree.K.). In the present invention, solar radiation in combination with a blackbody whose temperature is less than about 1500.degree. K. will not cause a false alarm since the UV channel would provide no response.
Another prior art reference is that of U.S. Pat. No. 4,455,487 to Wendt. Wendt refers to a two channel fire detector system with an IR and UV ratio detector. The UV sensor is responsive to UV radiation over the wavelength range 190 nanometers to 270 nanometers while the IR sensor is responsive to IR radiation in a bandwidth of 4.1 microns to 4.7 microns. Signal processing electronics in each channel produce a normalized output signal proportional to the power of incident IR and UV radiation within specific bandwidths. The system features a ratio detector that repeatedly forms a ratio of the normalized IR and UV inputs and compares the ratio to a known range of values, the known range of values being characteristic of fires. A discriminator connected to the output of the ratio detector generates a fire alarm signal only if the majority of these ratio comparisons are fire indicating. The system also includes a feedback loop in the IR processing channel that automatically adjusts the output of the channel to compensate for time varying background IR radiation such as sunlight.
Another prior art fire detector is disclosed by U.S. Pat. No. 4,463,260 to Ikeda. Ikeda discloses a flame detector comprised of three IR channels. One IR sensor is responsive to a band of radiation centered near 2.5 microns. The second IR sensor is responsive to a band of radiation centered near 3.5 microns. The third IR sensor is responsive to a band of radiation centered near 4.3 microns. Successful detection of a hydrocarbon flame depends upon sensing the larger signals from both the 2.5 micron and 4.3 micron channels in relation to the magnitude of the signal sensed by the 3.5 micron channel. One problem with this scheme is the inability to detect flames from non-hydrocarbon fuels which show no emission near 4.3 microns. In addition, it appears that a heated object whose temperature is less than 1000.degree. K. which produces a relatively high irradiance level, in combination with solar radiation, could produce a false indication of fire. Additionally, if water or ice cover the 2.5 micron IR sensor, a hydrocarbon flame would not be detected since response from that sensor would be very small. Another disadvantage of Ikeda is that it requires three discrete sensor channels to detect a hydrocarbon fire, as opposed to the present invention, which requires only two sensor channels.
The final prior art reference is the Model FS2000 Multispectrum Optical fire sensor system, manufactured by The Fire Sentry Corp., 1401A Warner Ave, Tustin, Calif. 92680 (patent pending). The FS2000 has a three channel optical fire detection system having three optical sensors. One sensor is responsive to UV radiation within the spectral band 185 nanometers to 260 nanometers. The second sensor is responsive to visible radiation within the spectral band 400 nanometers to 700 nanometers. The third sensor is responsive to radiation with the spectral band 0.7 to 3.5 microns.
One disadvantage of the FS2000 is that it will not be as sensitive to hydrocarbon flames as the present invention since the FS2000 does not sense the very strong carbondioxide emission band near 4.3 microns.
A second disadvantage with the FS2000 is that solar radiation combined with a source of UV radiation such as arc-welding could conceivably result in a false alarm or false indication of fire. This is because solar radiation is very strong throughout the visible spectral region (400 nanometers to 700 nanometers) and for most of the IR spectral region from 0.7 microns to 3.5 microns, with the exception of several atmospheric water vapor and carbon dioxide absorption bands (the principal water absorption band resides in the 2.5 micron to 2.9 micron region where solar irradiance at sea level is negligible). Therefore, in the system of the FS2000, solar radiation would yield large responses from both the sensor channel responsive to 400 nanometers to 700 nanometers and the sensor channel responsive to 0.7 micron to 3.5 microns. Solar radiation would not generate a response from the UV channel, but, the presence of any UV emitting source such as an arc or spark in the presence of sunlight could activate all three channels, and result in a false alarm condition. It is conceivable that a false alarm condition may also result if a UV emitting source such as an arc or a spark is present in conjunction with a second radiating source which emits strongly in the spectral regions 400 nanometers to 700 nanometers as well as in the region from 700 nanometers to 3.5 microns. Many blackbody radiators could provide the function of this second radiating source.
A third disadvantage of the FS2000 fire detection system is that it requires the use of three discrete sensors in order to detect flame from hydrocarbon and non-hydrocarbon fuels. The present invention is less complex in that this result is achieved by use of only two discrete optical sensors, and, in the preferred embodiment, the IR sensor includes a single IR filter and a single active element.
Accordingly, it is the object of the invention to to create a fire detection system with the ability to respond to fires from both hydrocarbon or organically based fuels as well as from certain non-hydrocarbon or inorganically based fuels, including certain metals.
It is another object of the invention to create a fire detection system capable of detecting fires from hydrocarbon and non-hydrocarbon fuels with minimum interference from radiation emitted from non-fire sources including blackbody, solar radiation, arc-welding and lightning.
It is yet another object of the invention to create a fire detection system that maximizes the sensitivity of the fire detector to fire from certain non-hydrocarbon fuels, including, but not limited to, hydrogen and pure hydrazine. Also detected would be flame emission from certain metals including, but not limited to, magnesium, aluminum, sodium and potassium. Also included is the ability to detect flame from chemicals such as ammonia and silane, without increasing the response level of the fire detector to the non-fire or potential false alarm sources noted above.
It is still yet another object of the invention to create a fire detection system that has improved IR channel response to fire from clean-burning hydrocarbon fuels such as methane, ethane, propane, butane and their associated alcohols.
It is still yet another object of the invention to create a fire detection system that has improved IR channel response to fire from heavy hydrocarbon fuels (both long chain and cyclic types, both saturated and unsaturated) such as gasoline, kerosene, jet fuels, diesel fuel, benzene and toluene.
It is still yet another object of the invention to create a fire detection system that can accomplish the above identified objectives by using a two channel UV/IR system employing as few radiation sensors as possible, those sensors being of the simplest design possible.